Mouse model systems are used to determine how retroviruses containing nuclear oncogenes can transform hematopoietic cells. The myb-ets-containing ME26 virus encodes a 135 kD gag-myb- ets fusion protein that can bind to DNA. The virus causes a high incidence of leukemia in mice and can induce hematopoietic precursor cells to grow in vitro on the erythroid hormone, erythropoietin (Epo). The viral protein was previously shown to transcriptionally activate an erythroid-specific transcription factor, GATA-1, and then cooperate with the GATA-1 protein to transactivate the Epo receptor (EpoR) gene. Studies utilizing deletion mutants of the GATA-1 promoter inserted into a luciferase expression vector demonstrated that transactivation by ME26 virus is not limited to one region of the promoter. The minimal promoter was deleted to nucleotide -153, a region which contains no myb or ets consensus sites, but does contain two binding sites for the Sp1 transcription factor. We are currently testing the possibility that ME26 virus is indirectly activating the GATA-1 promoter by interacting with the Sp1 protein. In examining Epo-dependent cell lines derived from ME26 virus-infected mice, we have determined that the mono or oligo clonal insertion of the virus into the target cell DNA may occur at a preferred site. Genomic DNA libraries have been made in bacteriophage vectors, and we are examining several possible clones of virus-host junction fragments to determine what gene is being interrupted in ME26 virus-induced leukemia. Studies on the role which myb and ets sequences play in transactivation of the GATA-1 gene showed that v-myb and v-ets of ME26 must be expressed as a fusion protein to exert an effect on the promoter. It was also discovered that m-ets2 and v-myb are potent activators of the GATA-1 promoter, although their effects are not additive when they are transfected together. These results suggest that c-ets and myb genes may play a role in normal erythropoiesis.